This invention relates to implantable medical devices (IMDs) and their various components, including flat electrolytic capacitors for same, and methods of making and using same, particularly a simplified, miniature capacitor connector block and wiring harness utilizing an epoxy droplet and method of making same.
As described in the above-referenced parent application Ser. No. 09/104,104, and the provisional application that it claims priority from, a wide variety of IMDs are known in the art. Of particular interest are implantable cardioverter-defibrillators (ICDs) that deliver relatively high energy cardioversion and/or defibrillation shocks to a patient""s heart when a malignant tachyarrhythmia, e.g., atrial or ventricular fibrillation, is detected. Current ICDs typically possess single or dual chamber pacing capabilities for treating specified chronic or episodic atrial and/or ventricular bradycardia and tachycardia and were referred to previously as pacemaker/cardioverter/defibrillators (PCDs). Earlier developed automatic implantable defibrillators (AIDs) did not have cardioversion or pacing capabilities. For purposes of the present invention ICDs are understood to encompass all such IMDs having at least high voltage cardioversion and/or defibrillation capabilities.
Generally speaking, it is necessary to employ a DCxe2x80x94DC converter within an ICD implantable pulse generator (IPG) to convert electrical energy from a low voltage, low current, electrochemical cell or battery enclosed within the IPG housing to a high voltage energy level stored in one or more high energy storage capacitor, as shown for example, in commonly assigned U.S. Pat. No. 4,548,209. The conversion is effected upon confirmation of a tachyarrhythmia by a DCxe2x80x94DC xe2x80x9cflybackxe2x80x9d converter which includes a transformer having a primary winding in series with the battery and a secondary winding in series with the high energy capacitor(s) and an interrupting circuit or switch in series with the primary coil and battery that is periodically opened and closed during a charging cycle. Charging of the high energy capacitor is accomplished by inducing a voltage in the primary winding of the transformer creating a magnetic field in the secondary winding when the switch is closed. The field collapses when the current in the primary winding is interrupted by opening the switch, and the collapsing field develops a current in the secondary winding which is applied to the high energy capacitor to charge it. The repeated interruption of the supply current charges the high energy capacitor to a desired level of several hundred volts over a charging time of the charge cycle. Then, the energy is rapidly discharged from the high voltage capacitor(s) through cardioversion/defibrillation electrodes coupled to the IPG through ICD leads and arranged about or in a heart chamber or vessel if the tachyarrhythmia is confirmed as continuing at the end of the charge time. The cardioversion/defibrillation shocks effected by discharge of such capacitors are typically in the range of about 25 to 40 Joules. The process of delivering cardioversion/defibrillation shocks in this way may be repeated if an earlier delivered cardioversion/defibrilation shock does not convert the tachyarrhythimia to a normal heart rhythm.
Energy, volume, thickness and mass are critical features in the design of ICD pulse generators that are coupled to the ICD leads. The battery(s) and high voltage capacitor(s) used to provide and accumulate the energy required for the cardioversion/defibrilation shocks have historically been relatively bulky and expensive. Presently, ICD IPGs typically have a volume of about 40 to about 60 cc, a thickness of about 13 mm to about 16 mm and a mass of approximately 100 grams.
It is desirable to reduce the volume, thickness and mass of such capacitors and ICD IPGs without reducing deliverable energy. Doing so is beneficial to patient comfort and minimizes complications due to erosion of tissue around the ICD IPG. Reductions in size of the capacitors may also allow for the balanced addition of volume to the battery, thereby increasing longevity of the ICD IPG, or balanced addition of new components, thereby adding functionality to the ICD IPG. It is also desirable to provide such ICD IPGs at low cost while retaining the highest level of performance. At the same time, reliability of the capacitors cannot be compromised.
Various types of flat and spiral-wound capacitors are known in the art, some examples of which are described as follows and/or may be found in the patents listed in Table 1 of the above-referenced parent patent application Ser. No. 09/104,104.
Prior art high voltage electrolytic capacitors used in ICDs have two or more anode and cathode layers (or xe2x80x9celectrodesxe2x80x9d) and operate at room or body temperature. Typically, the capacitor is formed with a capacitor case enclosing an etched aluminum foil anode, an aluminum foil or film cathode, and a Kraft paper or fabric gauze spacer or separator impregnated with a solvent based liquid electrolyte interposed therebetween. A layer of aluminum oxide that functions as a dielectric layer is formed on the etched aluminum anode, preferably during passage of electrical current through the anode. The electrolyte comprises an ion producing salt that is dissolved in a solvent and provides ionic electrical conductivity between the cathode and the aluminum oxide dielectric. The energy of the capacitor is stored in the electrostatic field generated by opposing electrical charges separated by the aluminum oxide layer disposed on the surface of the anode and is proportional to the surface area of the aluminum anode. Thus, to minimize the overall volume of the capacitor one must maximize anode surface area per unit volume without increasing the capacitor""s overall (i.e., external) dimensions. The separator material, anode and cathode layer terminals, internal packaging, electrical interconnections, and alignment features and cathode material further increase the thickness and volume of a capacitor. Consequently, these and other components in a capacitor and the desired capacitance limit the extent to which its physical dimensions may be educed.
Some ICD IPGs employ commercial photoflash capacitors similar to those described by Troup in xe2x80x9cImplantable Cardioverters and Defibrillators,xe2x80x9d Current Problems in Cardiology, Volume XIV, Number 12, December 1989, Year Book Medical Publishers, Chicago, and as described in U.S. Pat. No. 4,254,775. The electrodes or anode and cathodes are wound into anode and cathode layers separated by separator layers of the spiral. Anode layers employed in such photoflash capacitors typically comprise one or two sheets of a high purity (99.99%), porous, highly etched, anodized aluminum foil. Cathode layers in such capacitors are formed of a non-porous, highly etched aluminum foil sheet which may be somewhat less pure (99.7%) respecting aluminum content than the anode layers. The separator formed of one or more sheet or layer of Kraft paper saturated and impregnated with a solvent based liquid electrolyte is located between adjacent anode and cathode layers. The anode foil thickness and cathode foil thickness are on the order of 100 micrometers and 20 micrometers, respectively. Most commercial photoflash capacitors contain a core of separator paper intended to prevent brittle, highly etched aluminum anode foils from fracturing during winding of the anode, cathode and separator layers into a coiled configuration. The cylindrical shape and paper core of commercial photoflash capacitors limits the volumetric packaging efficiency and thickness of an ICD IPG housing made using same.
The aluminum anodes and cathodes of aluminum electrolytic capacitors generally each have at least one tab extending beyond their perimeters to facilitate electrical connection of all (or sets of) the anode and cathode layers electrically in parallel to form one or more capacitor and to make electrical connections to the exterior of the capacitor case. Tab terminal connections for a wound electrolytic capacitor are described in U.S. Pat. No. 4,663,824 that are laser welded to feedthrough pin terminals of feedthroughs extending through the case.
Wound capacitors usually contain two or more tabs joined together by crimping or riveting.
Flat electrolytic capacitors have also been disclosed in the prior art for general applications as well as for use in ICDs. More recently developed ICD IPGs employ one or more flat high voltage capacitor to overcome some of the packaging and volume disadvantages associated with cylindrical photoflash capacitors. For example, U.S. Pat. No. 5,131,388 discloses a flat capacitor having a plurality of stacked capacitor layers. each comprising an xe2x80x9celectrode stack sub-assemblyxe2x80x9d. Each capacitor layer contains one or more anode sheet forming an anode layer having an anode tab, a cathode sheet or layer having a cathode tab and a separator for separating the anode layer from the cathode layer. In the ""388 patent, the electrode stack assembly of stacked capacitor layers is encased within a non-conductive, polymer envelope that is sealed at its seams and fitted into a chamber of a conductive metal, capacitor case or into a compartment of the ICD IPG housing, and electrical connections with the capacitor anode(s) and cathode(s) are made through feedthroughs extending through the case or compartment wall. The tabs of the anode layers and the cathode layers of all of the capacitor layers of the stack are electrically connected in parallel to form a single capacitor or grouped to form a plurality of capacitors. The aluminum anode layer tabs are gathered together and electrically connected to a feedthrough pin of an anode feedthrough extending through the case or compartment wall. The aluminum cathode layer tabs are gathered together and electrically connected to a feedthrough pin of a cathode feedthrough extending through the case or compartment wall or connected to the electrically conductive capacitor case wall.
Many improvements in the design of flat aluminum electrolytic capacitors for use in ICD IPGs have been disclosed, e.g., those improvements described in xe2x80x9chigh Energy Density Capacitors for Implantable Defibrillatorsxe2x80x9d presented by P. Lunsmann and D. MacFarlane at CARTS 96: 16th Capacitor and Resistor Technology Symposium, 11-15 March 1996, and at CARTS-EUROPE 96: 10th European Passive Components Symposium. 7-11 October 1996, pp. 35-39. Further features of flat electrolytic capacitors for use in ICD IPGs are disclosed in U.S. Pat. Nos. 4,942,501; 5,086,374; 5,146,391; 5,153,820; 5,562,801; 5,584,890; 5,628,801; and 5,748,439, all issued to MacFarlane et al.
A number of recent patents including U.S. Pat. No. 5,660,737 and U.S. Pat. Nos. 5,522,851; 5,801,917; 5,808,857; 5,814,082; 5,908,151; 5,922,215; 5,926,357; 5,930,109; 5,968,210 and 5,983,472, all assigned to the same assignee, disclose related flat electrolytic capacitor designs for use in ICDs. In several of these patents, internal alignment elements are employed as a means for controlling the relative edge spacing of the anode and cathode layers from the conductive capacitor case. In these patents, each anode layer and cathode layer is provided with an outwardly extending tab, and the anode and cathode tabs are electrically connected in common to a feedthrough pin and a step feature of the conductive capacitor case, respectively. The cathode tabs are gathered together against the step feature and ultrasonically welded together and to the step feature. In the ""357 patent, the anode tabs are laser welded to one end of an aluminum ribbon that is ultrasonically welded at its other end to an aluminum layer that is ultrasonically welded to the terminal pin. The feedthrough terminal pin is electrically isolated from and extends outside and away from the case to provide an anode connection pin. A cathode connection pin is attached to the case and extends outwardly therefrom. The anode and cathode connection pins are electrically connected into the DCxe2x80x94DC converter circuitry, but the attachment mechanism is not described in any detail.
Other ways of sealing the capacitor cases and making electrical connections with the anodes and cathodes through or to the capacitor case are disclosed in the prior art. One construction employed from about 1960 to about 1985 and disclosed in U.S. Pat. No. 4,521,830 uses a plastic header with two molded-in threaded aluminum terminals of the type shown in U.S. Pat. No. 3,789,502, where plastic is molded around the terminals. An aluminum serrated shank terminal extending through a thermal plastic header is disclosed in U.S. Pat. Nos. 3,398,333 and 4,183,600. In both patents the aluminum terminal is resistance-heated to a temperature such that the length of the terminal is collapsed and the center diameter is increased to press the serrations into the melted plastic. A header design employing a compression-fit set of terminals disposed in a polymer header is also disclosed.
A glass-to-metal seal terminal connection with a tantalum outer ring being laser welded into an aluminum case is disclosed in U.S. Pat. No. 4,987,519. A resin casing that has been previously formed from epoxy, silicon resin, polyoxybenzylene, polyether etherkeytone, or polyether sulfone, and that has high heat resistance is disclosed in U.S. Pat. No. 4,663,824. Electrical anode and cathode connections are made via spaced apart terminals molded into and extending through the non-conductive resin casing.
It is desirable to simplify and minimize numbers of parts and manufacturing steps involved in making electrical, anode and cathode connections through the capacitor case to an exterior connector header and wiring harness for the anode and cathode electrical connections. It is also highly desirable particularly in IMD applications to minimize the size of the connector header and wiring harness while retaining a high level of safety and reliability over a relatively long time period of implantation.
The present invention provides for an elegant and miniaturized connector block and method of manufacturing the same for electrolytic capacitors especially designed for use in IMDs
In one embodiment, the capacitor comprises an electrode stack assembly and electrolyte are located within the interior case chamber of a hermetically sealed capacitor case. The electrode stack comprises a plurality of capacitor layers stacked in registration upon one another, each capacitor layer comprising a cathode layer having a cathode tab, an anode sub-assembly comprising at least one anode layer having an anode tab, and a separator layer located between adjacent anode and cathode layers, whereby all adjacent cathode layers and anode layers of the stack are electrically insulated from one another by a separator layer. Anode terminal means extend through the capacitor case side wall for electrically connecting a plurality of the anode tabs to one another and providing an anode connection terminal at the exterior of the case. Cathode terminal means extend through or to an encapsulation area of the capacitor case side wall for electrically connecting a plurality of the cathode tabs to one another and providing a cathode connection terminal at the exterior of the case. A connector assembly is electrically attached to the anode connection terminal for making electrical connection with the anode tabs and to the cathode connection terminal for making electrical connection with the cathode tabs.
In accordance with the invention, a connector block is formed of an epoxy droplet adhered to the encapsulation area of the capacitor side wall surrounding and encapsulating the anode and cathode connection terminals and the electrical connections with the connector assembly.
The connector block is formed on an encapsulation area of the case side wall of epoxy that is cured for a period of time under elevated temperature conditions while rotating the capacitor assembly. The epoxy is applied in a liquid state, and the rotation and temperature causes the epoxy to flow into gaps of and to completely cover the anode and cathode terminal means and the electrical connections with the connector assembly, to drive air bubbles to the exposed surface, and to shape the exterior surface to a uniform, repeatable configuration.
At least one edge of the encapsulation area is preferably masked to restrict flow of epoxy out of the encapsulation area when it is applied and during curing.
In case neutral capacitors, the anode and cathode terminal means comprise a feedthrough assembly fitted into an opening in the encapsulation area of the capacitor case side wall. The anode and cathode feedthrough assemblies further comprise anode and cathode feedthrough pins coupled at an internal pin end with the plurality of anode and cathode tabs, respectively. Electrically insulating spacers support and electrically insulate the feedthrough pins from the capacitor case and disposing the external pin ends away from the case to provide the anode and cathode connection terminals. A portion of the epoxy droplet flows into the spacers around the terminal pins toward the interior case chamber to seal the feedthrough assemblies to prevent leakage of electrolyte. Preferably, exposed wire ends of the connector assembly are electrically and mechanically coupled to the external pin ends, preferably by cross-wire welding.
In one case negative capacitor embodiment, the cathode terminal means comprises means for electrically connecting the plurality of cathode tabs to the case side wall to provide the cathode connection terminal upon the case side wall within the encapsulation area. An exposed wire end of the connector assembly is attached to the case wall within the encapsulation area.
In a further case negative embodiment, the cathode terminal means comprises a cathode feedthrough pin extending through the case side wall having an internal pin end connected to the plurality of cathode tabs and an external pin end extending away from the case to provide the cathode connection terminal, the cathode feedthrough pin electrically connected with the case. Preferably, an exposed wire end of the connector assembly is electrically and mechanically coupled to the cathode external pin end, preferably by cross-wire welding.
The epoxy droplet provides a reliable, reproducible, inexpensive, attractive, miniaturized connection of the connector assembly with the anode and cathode tabs.